Door seal for microwave ovens



Allg- 25, 1970 n. B. HAAGENs'L-:N ET Al. 3,525,841

DOOR SEAL FOR MICROWAVE OVENS Filed Nov. 4, 1968 INVENTORS DUANE 5.HAGENSE/V SAMUEL G. SWEET BY-gwu;

ATTOBN E Y United States Patent O 3,525,841 DOOR SEAL FOR MICROWAVEOVENS Duane B. Haagensen, Edina, and Samuel G. Sweet,

Hopkins, Minn., assignors to Litton Precision Products, Inc.,Minneapolis, Minn., a corporation of Dela- Ware Filed Nov. 4, 1968, Ser.No. 772,982 Int. Cl. H05b 9/ 06 U.S. Cl. 219-10.55 8 Claims ABSTRACT OFTHE DISCLOSURE An improved door seal for microwave ovens wherein aplurality of spacers provides a port of entry for electromagnetic waveenergy to enter a combination of an attenuating cavity and an absorptioncavity which together provide a volumetric resonant cavity that presentshigh empedance to electromagnetic wave energy within the attenuatingcavity and a terminating impedance to electromagnetic wave energy withinthe absorption cavity.

FIELD OIF THE INVENTION This invention relates to microwave ovens andmore particularly to a novel door seal for preventing the escape ofelectromagnetic wave energy from such ovens.

DESCRIPTION OF THE PRIOR ART It is well known that electromagnetic Waveenergy may be utilized for heating foodstuff or other lossy dielectricmaterials. The foodstuff or other materials are placed in a cavity of amicrowave oven and are exposed to electromagnetic wave energy that issupplied by a suitable source, e.g., a magnetron. In order to facilitatethe insertion and the removal of foodstuff to and from the cavity of themicrowave oven, a movable door is provided for such ovens. 'I'heinterior surface of the movable door completes the cavity to ensureproper heating of the foodstuff and prevents the wholesale escape ofelectromagnetic wave energy from the oven. In addition, door seals mustbe provided to prevent any further leakage of electromagnetic waveenergy from the oven.

In the past, microwave oven manufacturers have utilized various types ofdoor seals. Originally, microwave ovens featured direct contact betweenthe inside surface of the metallic door and the front surface of themetallic housing, with the addition of spring biasing of the door tomaintain the door seal. However, the spring biasing method did not provesatisfactory over a period `of time, in some cases, because of warpingof the metallic surfaces or other damage thereto, and in other cases,because of the deterioration of the springs. In either case, arcingbetween the respective surfaces of the door and the housing andexcessive leakage from the oven occurred, unless proper maintenanceprocedures were strictly observed.

Other approaches attempted by microwave oven manufacturers included theinsertion of a number of nonmetallic spacers or a conductive rubbergasket between the door and the microwave oven housing to establish agap so as to prevent arcing of the electromagnetic wave energy. One ofthese approaches included the placing of a cavity having a terminatingconducting surface located an integral number of one-half wave lengthsfrom the origin of the gap. -In some instances, such cavities have beenfilled with a dielectric material, which acts to shorten the physicallength of such cavities, in order to make the door seal more compact,while still electrically maintaining the one-half wave length betweenthe origin of the gap and the terminating conducting surface of thecavity. In either case, the cavity presented a short circuit which wasreflected back as a short circuit between the parallel platetransmission line formed by the interior surface of 3,525,841 PatentedAug. 25, 1970 ICC door and front surface of the housing. Thus,electromagnetic wave energy was blocked from leaving the oven cavity.However, either warping of the metallic surfaces or eventual wearing ofthe non-metallic spacers caused arcing between the metallic surfaces. Inthe case of the conductive rubber seal, the burning out of the rubberseal occurred.

Accordingly, it is an object of the present invention to provide animproved door seal f or microwave ovens which eliminates the possibilityof arcing of electromagnetic wave energy in proximity of the door seal.

It is a further object of the present invention to provide an improveddoor seal for preventing the escape of electromagnetic wave energythrough the exterior junction of the `door and the housing of microwaveovens.

It is a still further object of the present invention to provide animproved door seal for microwave ovens which includes a first cavity anda second cavity for attenuating and absorbing electromagnetic waveenergy, respectively.

\It is another object of the present invention to provide an improveddoor seal for microwave ovens which includes novel cavity means forpreventing the escape of electromagnetic wave energy from such ovens andrcduces the Weight of the door of such ovens.

SUMMARY OF THE INVENTION In accordance with the objects set forth above,the present invention provides an improved `door seal for microwaveovens comprising a plurality of spacers to providea gap for preventingarcing of electromagnetic wave energy between the respective surfaces ofthe door and the housing of the oven, the gap ensures a port of entryfor the electromagnetic wave energy, and a combination of anlattenuating cavity and an absorption cavity provides a volumetricresonant cavity that presents high impedance to electromagnetic waveenergy within the attenuating cavity and a terminating impedance toelectromagnetic wave energy within the absorption cavity.

BRIEF DESCRIPTION OF THE DRAWINGS i accordance with the presentinvention;

FIG. 2 isa cross sectional view of the microwave oven taken along theline 2 2 of FIG. l;

FIG. 3 is an alternate embodiment of the present invention in which thecavities have a vaporized metallic coating thereon; and

FIG. 4 is an enlarged view of one of the corners of the door of themicrowave oven of FIG. 1 with part of the door cut away to show theelements of a door seal in accordance with the present invention.

DESCRIPTION OF THE PREFERRED` EMBODIMENTS provided to enable viewingL ofthe cooking of any foodstuff which may be within the microwave oven 10.Various controls are shown above the door 12; however, since suchcontrols are conventional and do not relate to the present invention,the controls are not described or illustrated further herein.

Referring now to FIG. 2, there is shown a cross sectional view of themicrowave oven 10 taken along the line 2-2 of FIG. 1. FIG. 2 illustratesa door seal for preventing the escape of electromagnetic wave energyfrom the microwave yoven 10. As further illustrated by FIG. 4, the doorseal is located completely around the perimeter of the interior surfaceof the door 12. The door 12 is constructed of metal castings '116 and18, preferably aluminum to maintain a light weight door which may beheld together by a number of conventional means, such as, a screw 19.The spaces 27 located between metal castings 16 and :18 house themechanism, not shown, for operating the handle 13, or are empty so as toreduce the weight of the door 12. Also shown in FIG. 2 is the housing 11having three surfaces 11b, 11C and 11d. The surface 11b and the window14 dene the location of a microwave oven cavity 111a, or the interior ofthe microwave oven 10. The numeral 17 designates the exterior panel ofthe door 12.

Within the metal casting 18, as defined by the surfaces 24a, 24b, 24Cand 24d, is an attenuating cavity 24 which is filled with a dielectric,preferably polypyroleyne. Utilization of the polypyroleyne filler in theattenuating cavity 24 allows the cavity to be physically smaller than ifthe cavity only contained air and further provides for attenuating anyelectromagnetic wave energy entering the attenuating cavity 24. Alsowithin the metal casting 18 is an absorption cavity 22 which is filledwith an electromagnetic wave energy absorbing material, for example,ferrite. The numeral 23 defines the surface of the absorption cavity 22that is adjacent the attenuating cavity 24. A retaining strip 20 isaffixed between the metal castings 16 and 18 and retains the ferrite inthe cavity 22, as shown. 'Ihe retaining strip 20 includes spacers 21 atthe four corners of the door 12, as illustrated in FIGS. 2 and 4. Thefour spacers 21 ensure that a proper gap is maintained between theinterior surface of the door 12 and the surface 11c of the housing 11.

In combination, the attenuating cavity 24 and the absorption cavity 22provide a volumetric resonant cavity such that there is a high impedanceto electromagnetic wave energy within the attenuating cavity 24 forattenuating such electromagnetic wave energy, and there is a terminatingimpedance to electromagnetic wave energy within the absorption cavity 22for absorbing such electromagnetic wave energy. Furthermore, a port ofentry 26, which has been established by the four spacers 21, ensuresthat a physical opening of such a capacitance to provide minimumimpedance to electromagnetic wave energy is maintained. Thus, theestablishment of a transmission line into the volumetric resonantcavity, or resonant cavity choke, is such, that it allows purposefulcapacity transmission into the volumetric resonant cavity.

In the operation of door seal illustrated in FIG. 2, the surface 11C ofthe .housing 11 and the interior surface of the window 14 define a gapwhich establishes the port f entry 26. Since an electromagnetic waveenergy pattern is established within the cavity 11a, a portion of the.electromagnetic wave energy will travel to the port of entry 26. Anyelectromagnetic wave energy within the cavity 11a that is directedthereto is allowed to enter. Upon entering the attenuating cavity 22,the electromagnetic wave energy therein will be attenuated. A majorportion of this attenuated electromagnetic wave energy will then enterthe absorption cavity 24 which maintains a terminating impedance toabsorb such electromagnetic energy. Thus, instead of providing a shortcircuit at the port of entry 26 which facilitates possible arcing, thepresent invention provides a port of entry 26 which allowselectromagnetic wave energy to enter; therefore, eliminating thepossibility of arcing. It was found that if the absorption cavity 22 isplaced as near as possible to a port of exit 25, maximum absorption ofany attenuated electromagnetic wave energy attempting to leave saidattenuating cavity 24 is accomplished by the absorption cavity 22.

In the practice of this invention, it was found that an effective sealwas constructed which maintained a radiation leakage level of less than1 rnw./cm.2 under all operating conditions. The constructed door seal,which was maintained entirely around the interior perimeter of the door12, included the cavity 24 having a polyproleyne filler therein, withthe polyproleyne filler 'being of the following dimensions at thefollowing respective surfaces: 2411:.480, +1100, .005 inch; 240:.900,-[-.000, 005 inch; 24d:.590; and 25:.240 inch. The ferrite material incavity 22 had a width of .1251.015 inch at surface 23 and a depth of.30811020 inch. In addition, it was found that if the opening is.0153005 at the port of entry 26, no arcng is experienced and the portof entry 26 thereby established allows the electromagnetic wave energyto enter the attenuating cavity 24. As is well known in the art, one ofthe standard operating frequencies for magnetrons employed in microwaveovens is 2450 megacycles per second. The door seal in accordance withthe present invention provides a volumetric resonant cavity exhibitingbroad 'band characteristics that effectively prevents the escape ofelectromagnetic wave energy from the microwave oven 10.

FIG. 3 illustrates another embodiment in accordance with this inventionwherein the attenuating cavity 24 and the absorption cavity 22 may beprovided with a vaporized coating of conductive material 28, forexample, aluminum, that the volumetric resonant cavity will be effectiveand at the same time will enable the elimination of a substantialportion of the metal casting 18 so as to provide a relatively lightweight door.

Thus, although the lpresent invention has been shown and described withreference to particular embodiments, for example, an attenuating cavityof a defined shape having polyproleyne therein, or a microwave ovenproviding a seal having its major components within the door,nevertheless, various changes and modifications obvious to a personskilled in the art to which the invention pertains, for example,attenuating cavities of a different physical size having otherdielectric material therein, or a microwave oven providing a seal havingits major cornponents within the housing are deemed to lie within thespirit, scope, and contemplation of the invention as set forth in theappended claims.

What is claimed is:

1. Microwave oven apparatus comprising:

a housing;

a heating cavity located within said housing, said heating cavity havingan opening therein;

means for supplying electromagnetic wave energy to said heating cavity;

a movable door mounted to said housing, said movable door in the closedposition completing with said heating cavity an effective heating cavityfor said electromagnetic wave energy supplied to said heating cavity;

spacing means for providing a physical gap between said movable door andthe surface immediately adjacent said movable door, said gap having aport of entry to allow electromagnetic wave energy to enter said gap;and

volumetric resonant cavity means located adjacent said gap and inproximity of the entire perimeter of the interior surface of saidmovable door for ensuring an electrical opening of such a capacitancethat minimum impedance to said electromagnetic wave energy occurs atsaid port of entry, said volumetric resonant cavity means includesattenuation cavity means for attenuating said electromagnetic waveenergy entering said gap, said attenuating cavity means having a port ofexit, said volumetric resonant cavity means further includes absorptioncavity means having a surface abutting said attenuating cavity means forabsorbing said attenuated electromagnetic wave energy, said absorptioncavity means being located in proximity of said port of exit of saidattenuating means. L

2. Microwave oven apparatus as recited in claim 1 wherein saidvolumetric resonant cavity means comprises:

a first cavity filled with a dielectric material for attenuating saidelectromagnetic wave energy; and

a second cavity filled with an absorption material for absorbing saidelectromagnetic wave energy.

3. Microwave oven apparatus as recited in claim 2 wherein saiddielectric materaial is polyproleyne and said absorption material isferrite.

4. Microwave oven apparatus as recited in claim 1 wherein saidvolumetric resonant cavity means is located within said movable door.

5. Microwave oven apparatus as recited in claim 1 wherein saidvolumetric resonant cavity means comprises in combination:

a dielectric material for attenuating said electromagnetic wave energy;

an absorption material for absorbing said electromagnetic wave energy,said absorption material having a surface abutting said dielectricmaterial; and

a vaporized coating of conductive material enclosing said dielectricmaterial and said absorption material except at the surface of saiddielectric that is exposed to said gap.

6. Microwave oven apparatus as recited in claim 5 wherein saiddielectric material is polyproleyne, said absorption material isferrite, and said conductive material is aluminum.

7. Microwave oven apparatus as recited in claim 1 wherein the width ofsaid port of entry is approximately .015 inch.

8. Microwave oven apparatus comprising:

a housing;

a heating cavity located within said housing, said heating cavity havingan opening therein;

means for supplying electromagnetic wave energy at a frequency ofapproximately 2450 megacycles to said heating cavity;

a movable door mounted to said housing, said movable door in the closedposition completing with said heating cavity an effective heating cavityfor said electromagnetic wave energy supplied to said heating cavity;

spacing means for providing a gap between said mofvable door and thesurface immediately adjacent said movable door, said gap having a portof entry ofI a width of approximately .015 inch to allow electromagneticwave energy to enter said gap; and

volumetric resonant cavity means, located adjacent said gap andincluding polyproleyne filled attenuating cavity means and ferritefilled absorption cavity means, for attenuating and absorbing saidelectromagnetic wave energy traveling through said gap and that enterssaid volumetric lresonant cavity, said ferrite being located in closeproximity of said gap and having a surface abutting said polyproleyne.

References Cited UNITED STATES PATENTS 2,958,754 ll/1960 Hahn 2l9-10.553,182,164 5/1965 Ironfield 2l9-l0.55 3,196,242 7/1965 De Vries et al2l9l0.55 3,249,731 5/1966 Johnson 2l9-l0.55 3,351,730 11/1967 Pahlman2l9l0.55

JOSEPH V. TRUNE, Primary Examiner L. H. BENDER, Assistant Examiner img?UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No.315251841 Dated August 2751 1970 Invencods) Duane B. Hclagensen undSamuel G. Sweet It is certified that error appears in theabove-idetified patent and that said Letters Patent are hereby correctedas shown below:

r-Col. l, line 18, "empedanceI should be --impedcmce j Col. 3, line 22,lpolypyrolene" should be polyproleyne Col. 3, line 23, "polypyrolene"should be --polyproleyne Col. 4, line ll, the figure ".900" should be.990

SIGNED @man m5 1971 Ism) Attest: 2

Edward M. Hawker. Ii'- m *usgang n. mum om omissioner at Pam

